If you aren't seeing the frequency on the cart page, similar to the screenshot below, it is because your theme does not display them yet.
In order to fix this, just include the following line of code in your cart-template.liquid file (or similar file) in the for loop for your line items.
{%- if item.selling_plan_allocation -%}
<p>{{item.selling_plan_allocation.selling_plan.name}}</p>
{%- endif -%}
This section will display the selling plan name (aka frequency) to the customer on the cart page. Newer themes will already have this taken care of, so if you are using a purchased theme, your provider should be able to provide you an update as well.
This code can be customized as needed. Here is an example from the native Shopify Debut theme, available to download in your Shopify admin.